During the fueling of vehicles, liquid gasoline is delivered into the vehicle fuel tank, and a mixture of gasoline vapor and air is displaced from the vehicle fuel tank. To minimize release of gasoline vapors into the atmosphere, gasoline dispenser nozzles are typically equipped with vapor recovery vacuum systems to collect the displaced gasoline vapor and air, and deliver it back into the ullage of the underground storage tank (UST). The ullage is the amount or volume by which the UST is short of being full of liquid. Preferably, a 1-to-1 ratio balance is sought between the volume of liquid gasoline drawn from the UST to the volume of gasoline vapor returned into the ullage by the vapor recovery system. This balance is difficult to achieve; resulting in air or excess amounts of gasoline vapor being introduced into the UST. This excess vapor will then be discharged through the UST pressure/vacuum (PV) valve (e.g., vent).
Ideally, the use of a PV valve on the UST vent prevents vapors from escaping into the atmosphere, while also preventing fresh air from entering into the UST during normal operation. Typically, the differential operational pressure settings commonly used on PV valves for gasoline storage are 8 inches of water column (WC) vacuum together with 3 inches of WC pressure in the UST. For example, when fueling a vehicle, the removal of gasoline from the UST causes the vapor space pressure to decrease to −8.0 inches WC, thus causing the UST PV valve to open, drawing fresh air into the UST.
Later, when fueling activity slows down in the evening, the large quantity of air previously drawn into the UST will promote evaporation of liquid gasoline into the air in the ullage. As the enclosed system of gas and liquid moves toward vapor pressure equilibrium, the hydrocarbon concentration in the ullage reaches a balance with the volume of liquid gasoline in the UST. This tendency toward equilibrium of gasoline liquid and vapor will cause the pressure in the UST to rise, potentially to a positive pressure of +3.0 inches W.C., which will cause the PV valve to open, releasing gasoline vapors into the environment.
Current PV valve designs typically consist of two weighted discs suspended by springs. In this design, the area of the disc exposed to the differential pressure from the UST and the spring tension are used to determine the set points of the PV valve. One disc assembly is used for pressure, the other for vacuum. Leakage through the PV valve is minimized when the differential pressure is within the PV valve operational settings (i.e., 8 inches of WC vacuum together with 3 inches of WC pressure).
Periodic testing of PV valves at fueling facilities has shown that a high number of PV valves fail to operate within the required regulatory and design parameters. The most common cause of this failure occurs when the seal fails to seat properly, thus allowing vapors to be released from, or air to enter into, the UST. On other occasions, the PV valves will fail to open at the operational settings for various reasons.
What is needed in the art is a specially designed valve that will prevent the UST ullage vapors from escaping to the atmosphere while still maintaining the proper operational differential pressure settings.